Embodiments presented herein relate to residual life assessment of airfoils such as turbine rotors.
Turbine and compressor airfoils operate under extreme conditions and may develop cracks during normal operating conditions. The cracks may develop due to high cycle fatigue, especially when a natural frequency of the airfoil matches the frequency of an excitation force experienced during normal turbomachinery operation. Power generation turbomachinery is typically exposed to external disturbances that have frequencies of 50/60 Hz and integral multiples of such frequencies. Airfoils used in such power generation machinery are designed to have a natural frequency different from the typical frequencies of external disturbances.
Apart from known external disturbances, the turbomachinery may experience unanticipated excitations that match airfoil frequencies in normal service. Such unanticipated excitations may cause the airfoil to resonate, thus exposing the airfoil to elevated amounts of vibration. Such vibration may cause cracks to form on airfoils or airfoil supporting elements such as the platform and dovetails. The cracks may propagate and rapidly extend due to the high cycle fatigue and vibration. Neglected cracks in the turbomachine airfoils may lead to unplanned outages.
Some known residual life assessment techniques to predict crack propagation rates are based on observed crack data obtained by periodic inspections. Such methods use statistical models, probabilistic models, and interpolation techniques based on observed data for obtaining expected future crack dimensions. Some of these types of methods do not always yield sufficiently accurate results. Often these types of methods may also not provide information about the underlying mechanism causing damage to a structural component, which may be critical in extending the residual life of the structural component.
Some residual life assessment techniques for low cycle fatigue and creep fatigue compute the crack growth rate based on the static response of the airfoil under a static load such as, for example, a time-invariant centrifugal load, steady-state airfoil metal temperature, and steady-state gas pressure on airfoil surface. Such techniques do not account for high cycle fatigue conditions, thus making such techniques unsuitable for assessing residual life under high cycle fatigue conditions.
Some other known residual life assessment techniques for high cycle fatigue may employ forced vibration response analysis to arrive at more accurate assessments. However, such techniques are computationally intense and often require large amounts of computation capacity.
Thus, there is a need for methods and systems which overcome these and other shortcomings associated with the known solutions.